The present invention relates to a gas laser apparatus of the type in which a gas contained in a dielectric discharge tube is excited by a high-frequency oscillator.
The high-frequency exciting system has recently occupied the attention of laser researchers and manufacturers as a preferable exciting system for gas laser apparatus. This is because the high-frequency exciting system possesses a great advantage over a direct-current exciting system by virtue of the features itemized below.
(1) Electrodes can be provided on the outside of a discharge tube. Since the electrodes do not directly contact a gas contained in the discharge tube, the gas is substantially free from deterioration. This arrangement of the electrodes is particularly suitable for use with carbon dioxide lasers in which carbon dioxide having a high chemical reactivity is employed.
(2) The discharge takes place longitudinally and the distance between the opposed electrodes can be narrowed so that the operating voltage is about one-tenth as large as the operating voltage of the direct-current exciting system. With this lower operating voltage, a solid-state power source is available.
(3) An improved distribution and uniformity of discharge provides a stable power output and an excellent laser mode.
(4) A high energy conversion efficiency due to a reactive ballast.
Owing to the foregoing advantageous features, a gas laser apparatus employing the high-frequency exciting system is smaller in size and higher in efficiency and quality than the gas laser apparatus utilizing the direct-current exciting system. The high-frequency excited gas laser apparatus find use in high precision, carbon dioxide laser cutting machines.
An example of such high-frequency excited gas laser apparatus is disclosed in Japanese Patent Laid-open Publication No. 60-3170.
The disclosed high-frequency excited laser apparatus includes, as reillustrated here in FIG. 3 of the accompanying drawings, a hollow cylindrical discharge tube 1 formed of a dielectric material such as SiO.sub.2 or Al.sub.2 O.sub.3, and a pair of metal electrodes 2, 2' extending helically around an the outer peripheral surface of the dielectric discharge tube 1 in parallel spaced relation to one another. The electrodes 2, 2' are generally formed by a printing process or a metal deposition process into a thin film layer. The metal electrodes 2, 2' are connected via a matching circuit 4 to a high-frequency power source 5. The high-frequency power source 5 applies a high-frequency voltage across the metal electrodes 2, 2' to excite a gas contained in the dielectric discharge tube 1 whereby a discharge of the gas takes place in a discharge space within the discharge tube 1.
The discharge condition thus produced by the conventional gas laser apparatus is likely to vary with the shape and arrangement of the electrodes and sometimes occurs only locally. As a result, the output power of the gas laser apparatus is lowered and the power output stability of the gas laser apparatus is considerably deteriorated too. In the worst case, the discharge terminates soon.
Prior attempts proposed to overcome the foregoing drawbacks include an improved arrangement of the electrodes in which the spacing between the electrodes is reduced or narrowed in such a manner as to enlarge a discharge region until a non-discharge region substantially eliminated. Such arrangement of the electrodes is not satisfactory because narrowing of the inter-electrode spacing has a limit, or a dielectric breakdown occurs through an air gap between adjacent portions of the electrodes when a high electric voltage in the order of several kV is applied across the closely spaced electrodes.